This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.
The present disclosure relates generally to methods and systems method and system for processing sonic data measured with a downhole tool. In particular, the present disclosure relates to methods and systems for estimating anisotropic properties of formation from acoustic data measured with a downhole tool such as a LWD (logging-while-drilling), MWD (measurements while drilling) tool or wireline logging tool.
In drilling or logging applications, acoustic measurements can be used to measure characteristics of the surrounding formation. Acoustic measurement techniques generally involve sensing acoustic waves generated by one or more acoustic sources and that have propagated through a geological formation. A geological formation whose rock properties are the same in all directions is an “isotropic” formation. A physically more precise description is “anisotropy”, i.e., a predictable variation of a physical property of a material with the direction in which it is measured. This “anisotropic” formation is a geological formation with directionally dependent properties and commonly caused by anisotropy of the constituting crystals, alignment of oblate particles, fine-scale layering or aligned and quasi-aligned fractures.
Several techniques for estimating the anisotropic properties of formation using sonic data are conventionally known, for example, as described in the documents of: X. Zeng. and C. Macbeth, “ALGEBRAIC PROCESSING TECHNIQUE FOR ESTIMATING SHEAR-WAVE SPLITTING IN NEAR-OFFSET VSP DATA: THEORY”, Geophysical Prospecting, 41(8), pp. 1033-1066, November 1993; B. Nolte and A. C. H. Cheng., ESTIMATION OF NONORTHOGONAL SHEAR WAVE POLARIZATIONS AND SHEAR WAVE VELOCITIES FROM FOUR-COMPONENT DIPOLE LOGS”, 1996, ERL Industry Consortia Technical Reports; and S. Bose, “Performance bounds on shear anisotropy azimuth estimation using borehole sonic logging tools”, Sensor Array and Multichannel Signal Processing Workshop Proceedings, 2002, the contents of which are incorporated herein in their entirety by reference thereto.
In such technique, for example, a numerical rotation method of Alford is used for estimating anisotropic properties of formation. This method is commonly referred to “Alford rotation” and a processing technique to project formation shear data measured in any two orthogonal directions into the fast and slow shear directions in the presence of shear-wave anisotropy, as described in the document of Alford, R. M., “Shear Data in the Presence of Azimuthal Anisotropy: Dilley, Tex.”, Expanded Abstracts, 56th SEG Annual International Meeting and Exposition, Houston, Tex., USA, Nov. 2-6, 1986, pp. 476-479, the content of which is incorporated herein in its entirety by reference thereto.
There is a need, however, for improving the technique for estimating the anisotropic properties of formation using sonic data.